Enhanced intraluminal flow measurement system using reference and combined signals

ABSTRACT

An enhanced intraluminal flow measurement system and method is conducive for a low-power ultrasonic system that can use continuous-wave (CW) Doppler sensing and wireless RF telemetry. Applications include measurement of blood flow in situ in living organisms. Implementations include an extraluminal component located outside of a body, such as a human or animal body, containing a lumen. The extraluminal component can be wirelessly coupled via an RF magnetic field or other RF field to an implantable intraluminal component. The intraluminal component (i.e. implant) is implanted inside of the lumen of the body such as a heart or elsewhere in a vasculature (such as in a dialysis shunt). The intraluminal component can telemeter, via RF electromagnetic signals, flow data directly out of the body housing the intraluminal component to be received by the extraluminal component.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally related to flow measurement.

2. Description of the Related Art

Conventional methods for measuring intraluminal flow, such as bloodflow, include use of continuous wave Doppler methods in which atransducer transmits an ultrasonic signal and another transducerreceives echoes from the sound reflecting off of surfaces moving alongwith the fluid. These conventional systems are generally more responsiveto a velocity component of the flow that is along a midline between thetwo transducers.

Conventional approaches include a first intraluminal transducerconfiguration 10, which positions a transmit transducer 12 and a receivetransducer 14 on opposite sides of a lumen 16 (such as in an inner areaof a blood vessel or other biological structure of a biological body) asshown in FIG. 1. With these conventional approaches the transmittransducer 12 and the receive transducer 14 are symmetrically alignedwith a first fluid flow 18 a and/or an oppositely directed second fluidflow 18 b, which both collective are known herein as fluid flow 18. Thetransmit transducer 12 sends out a transmitted beam 19 that is reflectedoff of surfaces 20 traveling in the first fluid flow 18 a and/or thesecond fluid flow 18 b through a sampling region 22 as a reflected beam24 to be received by the receive transducer 14.

Alignment of the transmit transducer 12 and the receive transducer 14 issuch that the transmitted beam 19 and the reflected beam 24 aredownstream of the transducers with respect to the first fluid flow 16and upstream of the transducers with respect to the second fluid flow 18as further shown in FIG. 1. By symmetrically positioning the transmittransducer 12 and the receive transducer 14 in the lumen, measurementscan be responsive to a flow vector component that is coaxial with anoverall longitudinal direction 26 of the lumen to increase sensitivityof the flow measurement.

Other conventional approaches include a second intraluminal transducerconfiguration 30 that positions the transmit transducer 12 and thereceive transducer 14 on a same side of the lumen 16 as shown in FIG. 2.In implementations, the transmitted beam 19 and the reflected beam 24are obliquely aimed to overlap into a version of the sampling region 22that can have a trapezoidal shape.

Unfortunately, in at least some cases, space to position fluid flowmeasuring devices intraluminally is rather limited, which tends torestrict the number and/or quality of the device components used.Consequently, measurement accuracy of fluid flow may be less thandesirable.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 is a sectional view of a first conventional positioning of atransmit transducers and a receive transducer for measuring intraluminalfluid flow.

FIG. 2 is a sectional view of a second conventional positioning of atransmit transducer and a receive transducer for measuring intraluminalfluid flow.

FIG. 3 is a schematic block diagram of an intraluminal component of afirst version of an enhanced intraluminal flow measurement system.

FIG. 4A is a schematic block diagram of a first extraluminal componentimplementation of the first version of enhanced intraluminal flowmeasurement system.

FIG. 4B is a schematic block diagram of a second extraluminal componentimplementation of the first version of the enhanced intraluminal flowmeasurement system.

FIG. 5 is a schematic block diagram of an intraluminal componentimplementation of a second version of the enhanced intraluminal flowmeasurement system.

FIG. 6A is a schematic block diagram of a first extraluminal componentimplementation of the second version of the enhanced intraluminal flowmeasurement system.

FIG. 6B is a schematic block diagram of a second extraluminal componentimplementation of the second version of the enhanced intraluminal flowmeasurement system.

FIG. 7 is a schematic block diagram of an intraluminal componentimplementation of a third version of the enhanced intraluminal flowmeasurement system.

FIG. 8 is a schematic block diagram of an extraluminal componentimplementation of the third version of the enhanced intraluminal flowmeasurement system.

DETAILED DESCRIPTION OF THE INVENTION

As discussed herein, implementations of an enhanced intraluminal flowmeasurement system and method is conducive for a low-power ultrasonicsystem that can use continuous-wave (CW) Doppler sensing and wireless RFtelemetry. Applications include measurement of blood flow in situ inliving organisms. Implementations include an extraluminal componentlocated outside of a body, such as a human or animal body, containing alumen. The extraluminal component can be wirelessly coupled via an RFmagnetic field or other RF field to an implantable intraluminalcomponent. The intraluminal component (i.e. implant) is implanted insideof the lumen of the body such as a heart or elsewhere in a vasculature(such as in a dialysis shunt). The intraluminal component can telemeter,via RF electromagnetic signals, flow data directly out of the bodyhousing the intraluminal component to be received by the extraluminalcomponent.

The implementations can allow a reduced number of electrical circuitcomponents having electrical power and space requirements more conducivefor insertion inside areas sized such as having a typical intravascularor intracardiac catheter diameter. Low power requirements can promote awider selection of power delivery alternatives. Consequently,continuous, real-time interrogation of fluid flow velocity usingultrasonic transducers is more readily available. Some implementationscan be placed in intravascular locations in animals or in the human bodyfor the purpose of measuring blood flow.

With some implementations a 1st signal representing a Doppler shift dueto fluid flow and a 2nd signal associated with an oscillator referenceoscillator can be simultaneously transmitted out of the body withoutinterfering with each other. With this approach complex (two-sided)Doppler spectrum is preserved to provide directional informationregarding the flow. Alternative implementations transmit a basebandDoppler signal out of the body by modulating an RF carrier to provide aone-sided Doppler spectrum with no direction information.

Implementations can be configured to sense fluid flow velocity, such asaverage velocity, without requiring precise aiming of an external datacollection device found with conventional approaches to increaseavailability for use in non-specialist clinics or in homes. Furthermore,fluid flow parameters can be automatically sampled periodically over anextended period of time in a fixed location within a predetermineddistance from the subject. Alternatively, fluid flow parameters can becontinuously sampled to provide trending data. Additionally,instantaneous fluid flow velocity can be sampled rapidly, to provide aflow velocity waveform, in a non-specialist environment.

An intraluminal component 100 of a first version of an enhancedintraluminal flow measurement system is shown in FIG. 3 as having anoscillator 102, an amplifier 104, an antenna-amplifier transmitter unit106, a frequency-selective filter 108, an amplifier 110, afrequency-selective filter 111, a mixer 112, and an antenna-amplifiertransmitter unit 114. In some implementations, the antenna-amplifiertransmitter unit 106 and the antenna-amplifier transmitter unit 114 canbe part of a single transmitter assembly. The oscillator 102 generatesthe 1st signal 116 having a depicted frequency, f_(C). The 1st signal istransmitted by the transmitter unit 106 to be received outside a bodycontaining the lumen 16. The 1st signal 116 is also amplified by theamplifier 104 and sent by the transmit transducer 12 as an ultrasonicsignal into the fluid flow 18 to be reflected off of surfaces 20 in thesampling region or area 22 as the reflected beam 24 to be received bythe receive transducer 14 and passed as an electrical signal to theamplifier 110 and outputted as a 2nd signal 118. As a result ofreflection of the 1st signal 116 off of the surfaces 20, the frequencyof the 2nd signal 118 is a Doppler shifted (either up-shifted ordown-shifted) version of the frequency of the 1st signal depending upondirection of the fluid flow 18 and placement of the transmit transducer12 and the receive transducer 14.

If the sampling region 22 is upstream of the transmit transducer 12 andthe receive transducer 14, then the frequency of the 2nd signal 118 willbe Doppler up-shifted as the frequency of the 1st signal, f_(C), plus aDoppler frequency shift, dF, according to principles of Doppler physics.If the sampling region 22 is downstream of the transmit transducer 12and the receive transducer 14, then the frequency of the 2nd signal willbe Doppler down-shifted as the frequency of the 1st signal, f_(C), minusthe Doppler frequency shift, dF, according to principles of Dopplerphysics. The transmit transducer 12 and the receive transducer 14 aredepicted in FIG. 3 as being in a generalized configuration 40 thatrepresents both instances in which the first intraluminal transducerconfiguration 10 is used and also represents instances in which thesecond intraluminal transducer configuration 30 is used.

The 2nd signal 118 is inputted into the frequency-selective filter 111and outputted as the 3rd signal 120 being selectively filtered having afrequency with a numerical fraction or numerical multiple of thefrequency of the 2nd signal. The frequency-selective filter 111 servesas a pass-through so that the frequency of the 3rd signal 120 can be thesame as the frequency of the 2nd signal 118. The oscillator 102 alsosends the 1st signal 116 to the frequency-selective filter 108, whichoutputs a 4th signal 122 having a frequency with a numerical fraction ornumerical multiple of the frequency of the 1st signal. In someimplementations, the frequency-selective filter 108 serves as apass-through so that the frequency of the 4th signal 122 can be the sameas the frequency of the 1st signal 116. The mixer 112 combines the 3rdsignal 120 and the 4th signal 122 to output a 5th signal 124. In someimplementations, the 5th signal 124 is a linear combination of the 3rdsignal 120 and the 4th signal 122 whereas in other combinations the 5thsignal is a non-linear combination of the 3rd signal 120 and the 4thsignal 122.

The 5th signal 124 is transmitted by the transmitter unit 114 to bereceived outside the body containing the lumen 16. In someimplementations, the frequency-selective filter 108 acts as a frequencydivider to divide the frequency of the 1st signal 116, the implant'slocal oscillator frequency, f_(C), by an integer N to produce the 4thsignal 122, which is mixed with the 3rd signal 120 by the mixer 112 tobe transmitted as the 5th signal 124 by the transmitter unit 114.

A first extraluminal component implementation 130 of the first versionof the enhanced intraluminal flow measurement system is shown in FIG. 4Aas having an antenna-amplifier receiver unit 132, a bandpass filter 134,a phase-lock loop 136, the voltage controlled oscillator (VCO) 138, abandpass filter 140, a mixer 142, a low-pass filter 144, a mixer 146, alow-pass filter 148, and a frequency transformation module 150. Examplesof suitable frequency transformations include discrete Fouriertransforms, real and complex fast Fourier transforms (FFT), fast Hartleytransforms, and related transforms and spectral estimators. The realtransforms provide only the single-sided spectrum, whereas the complextransforms provide the two-sided spectrum needed for directional flowmeasurement in situations where the flow can be in either direction. Insome implementations the antenna-amplifier receiver unit 132, thebandpass filter 134 and the bandpass filter 140 can be part of a singlereceiver assembly. The 1st signal 116 from the transmitter unit 106 andthe 5th signal 124 from the transmitter unit 114 are received by thereceiver unit 132 and passed on as a combined signal 152. The bandpassfilter 134 selects the 1st signal 116 out from the combined signal 152to pass on the 1st signal to the phase-lock loop 136.

The phase-lock loop 136 locks onto the 1st signal 116, which controlsthe VCO 138 in generating a 6th signal 154, which is an in-phase (0°)difference of the 4th signal 122 subtracted from the 1st signal 116 andin generating a 7th signal 156, which is a quadrature (90°) differenceof the 4th signal subtracted from the 1st signal. The mixer 142 combinesthe 5th signal 124 with the 6th signal 154 to produce the 8th signal158. The mixer 146 combines the 5th signal 124 with the 7th signal 156to produce the 9th signal 160. The low-pass filter 144 attenuates the8th signal 158 to produce the 10th signal 162.

The low-pass filter 148 attenuates the 9th signal 160 to produce the11th signal 164. The 10th signal 162 and the 11th signal 164 aredigitized and fed into the frequency transformation module 150, whichtransforms the signals from the time domain to the frequency domain.This results in a 12th signal 166 having a complex, two-sided Dopplerspectrum with accompanying two-sided axis, that discriminates betweenthe two types of flow velocities (flow toward and flow away from thereceive transducer 14) by placing each type on either side of thetwo-sided axis. Consequently, separation in measurement of these forwardand reverse flows can be accomplished.

A second extraluminal component implementation 170 of the first versionof the enhanced intraluminal flow measurement system is shown in FIG. 4Bas having an antenna-amplifier receiver unit 172 that passes the 1stsignal 116 from the transmitter unit 106 on to the phase-lock loop 136and an antenna-amplifier receiver unit 174 that passes the 5th signal124 from the transmitter unit 114 on to the mixer 142 and the mixer 146.In some implementations the antenna-amplifier receiver unit 172 and theantenna-amplifier receiver unit 174 can be part of a single receiverunit.

An intraluminal component 176 of a second version of the enhancedintraluminal flow measurement system is shown in FIG. 5 as having thefrequency-selective filter 108 coupled to the transmitter unit 106 tosend the 4th signal 122 outside of the lumen 16. For the intraluminalcomponent 176, the frequency-selective filter 111 is coupled to thetransmitter unit 114 to send the third signal 120 outside of the lumen16. In implementations where the frequency-selective filter 111 is apass-through, either the amplifier 110 or the amplifier of thetransmitter unit 114 need not be present.

A first extraluminal component implementation 180 of a second version ofthe enhanced intraluminal flow measurement system is shown in FIG. 6A ashaving an antenna-amplifier receiver unit 182, a bandpass filter 184, aphase-lock loop 186, the VCO 188, a bandpass filter 190, a mixer 192, alow-pass filter 194, a mixer 196, a low-pass filter 198, and a frequencytransformation module 200. Examples of suitable frequencytransformations include discrete Fourier transforms, real and complexfast Fourier transforms (FFT), fast Hartley transforms, and relatedtransforms and spectral estimators. The real transforms provide only thesingle-sided spectrum, whereas the complex transforms provide thetwo-sided spectrum needed for directional flow measurement in situationswhere the flow can be in either direction. In some implementations, theantenna-amplifier receiver unit 182, the bandpass filter 184, and thebandpass filter 190 can be a single receiver assembly. The 4th signal122 from the transmitter unit 106 and the 3rd signal 120 from thetransmitter unit 114 are received by the receiver unit 182 and passed onas a combined signal 202. The bandpass filter 184 selects the 4th signal122 out from the combined signal 202 to pass on the 4th signal to thephase-lock loop 186.

The phase-lock loop 186 locks onto the 4th signal 122, which controlsthe VCO 188 in generating a 13th signal 204, which is an in-phase (0°)or cosine version of the first signal 116 and in generating a 14thsignal 206, which is a quadrature (90°) or sine version of the firstsignal 116. The mixer 192 combines the 3rd signal 120 with the 13thsignal 204 to produce the 15th signal 208. The mixer 196 combines the3rd signal 120 with the 14th signal 206 to produce the 16th signal 210.The low-pass filter 194 attenuates the 15th signal 208 to produce the17th signal 212.

The low-pass filter 198 attenuates the 16th signal 210 to produce the18th signal 214. The 17th signal 212 and the 18th signal 214 aredigitized and fed into the frequency transformation module 200, whichtransforms the signals from the time domain to the frequency domain.This results in a 19th signal 216 having a complex, two-sided Dopplerspectrum with accompanying two-sided axis, that discriminates betweenthe two types of flow velocities (flow toward and flow away from thereceive transducer 14) by placing each type on either side of thetwo-sided axis. Consequently, separation in measurement of these forwardand reverse flows can be accomplished.

A second extraluminal component implementation 220 of the second versionof the enhanced intraluminal flow measurement system is shown in FIG. 6Bas having an antenna-amplifier receiver unit 222 that passes the 4thsignal 122 from the transmitter unit 106 on to the phase-lock loop 186and an antenna-amplifier receiver unit 224 that passes the 3rd signal120 from the transmitter unit 114 on to the mixer 192 and the mixer 196.In some implementations, the antenna-amplifier receiver unit 222 and theantenna-amplifier receiver unit 224 can be a single receiver assembly.

An intraluminal component 230 of a third version of the enhancedintraluminal flow measurement system is shown in FIG. 7 as includinglow-pass filter 232, a zero cross detector 234, an edge detector 236, anRF tank circuit 238, and a transmitter unit 240. The intraluminalcomponent 230 sends the fifth signal 124 from the mixer 112 to thelow-pass filter 232 to attenuate the sum-frequency-selective filtersfrom the mixer and is passed on to the zero cross detector 234 and thenon to the edge detector 236 to produce edge events. The edge events areused to trigger bursts of RF oscillation in the RF tank circuit 238,which is connected to the transmitter unit or assembly 240, whichtransmits a 20th signal 242 outside the lumen 16.

An extraluminal component implementation of the third version of theenhanced intraluminal flow measurement system is shown in FIG. 8 ashaving an antenna-amplifier receiver unit 252, an RF pulse detector 254,and a period counter 256. The receiver unit or assembly 252 receives the20th signal 242, which is passed on to the RF pulse detector 254 thatsends detected RF pulses contained in the 20th signal as a 21st signal.The period counter 256 measures the time intervals between the pulses ofthe 21st signal 258 and outputs a measure of the mean frequency of thesecond signal 118, which is the Doppler signal.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

1. For implantation into a biological structure having a fluid flow ofan average velocity with surfaces traveling with substantially theaverage velocity in the fluid flow through the biological structure, asystem comprising: an oscillator configured to generate a firstelectrical signal having a first frequency; a transmit transducerelectrically coupled to the oscillator to transmit an acoustic beambased upon the first electrical signal, the transmit transducerconfigured within the system to be positioned to transmit the acousticbeam into the fluid flow when the system is implanted into thebiological structure; a receive transducer configured within the systemto receive a reflected portion of the acoustic beam reflected off of aportion of the surfaces traveling in the fluid flow and to produce asecond electrical signal based upon the portion of the acoustic beamreceived by the receive transducer; a transmitter assembly electricallycoupled to the oscillator to transmit through the antenna anelectromagnetic signal based upon the first electrical signal; a mixerelectrically coupled to the receive transducer to receive a thirdelectrical signal based upon the second electrical signal, the mixerelectrically coupled to the oscillator to receive a fourth electricalsignal based upon the third electrical signal, the mixer configured togenerate a fifth electrical signal based upon the third electricalsignal and the fourth electrical signal received by the mixer; and thetransmitter assembly further including an antenna, the secondtransmitter unit electrically coupled to the mixer to transmit throughthe antenna an electromagnetic signal based upon the fifth electricalsignal.
 2. The system of claim 1 wherein the transmitter assemblyincludes: a first transmitter unit including an antenna, the firsttransmitter unit electrically coupled to the oscillator to transmitthrough the antenna the electromagnetic signal based upon the firstelectrical signal; and a second transmitter unit including an antenna,the second transmitter unit electrically coupled to the mixer totransmit through the antenna the electromagnetic signal based upon thefifth electrical signal.
 3. The system of claim 2 wherein the firsttransmitter unit and the second transmitter unit each further include anamplifier.
 4. The system of claim 1, further including a frequencyselective filter electrically coupled between the oscillator and themixer, wherein the fourth signal has a frequency substantially amultiple of the frequency of the first signal.
 5. The system of claim 4,further including a frequency selective filter electrically coupledbetween the oscillator and the mixer, wherein the fourth signal has afrequency substantially an integer multiple of the frequency of thefirst signal.
 6. The system of claim 1, further including a frequencyselective filter electrically coupled between the oscillator and themixer, wherein the fourth signal has a frequency substantially afraction of the frequency of the first signal.
 7. The system of claim 6,further including a frequency selective filter electrically coupledbetween the oscillator and the mixer, wherein the fourth signal has afrequency substantially an integer fraction of the frequency of thefirst signal.
 8. The system of claim 1 wherein the reflected portion ofthe acoustic beam received by the receive transducer has substantiallythe same frequency of the first electrical signal substantially Dopplershifted according to the average velocity of the fluid flow.
 9. Thesystem of claim 1, further including a frequency selective filterelectrically coupled between the receive transducer and the mixer,wherein the third signal has a frequency substantially a multiple of thefrequency of the second signal.
 10. The system of claim 9, furtherincluding a frequency selective filter electrically coupled between thereceive transducer and the mixer, wherein the third signal has afrequency substantially an integer multiple of the frequency of thesecond signal.
 11. The system of claim 9 further including an amplifierelectrically coupled between the frequency selective filter and thereceive transducer.
 12. The system of claim 1, further including afrequency selective filter electrically coupled between the receivetransducer and the mixer, wherein the third signal has a frequencysubstantially a fraction of the frequency of the second signal.
 13. Thesystem of claim 12, further including a frequency selective filterelectrically coupled between the receive transducer and the mixer,wherein the third signal has a frequency substantially an integerfraction of the frequency of the second signal.
 14. The system of claim1 wherein the reflected portion of the acoustic beam received by thereceive transducer has substantially the same frequency of the firstelectrical signal substantially Doppler shifted according to the averagevelocity of the fluid flow.
 15. The system of claim 1 wherein theacoustic beam transmitted by the transmit transducer has substantiallythe same frequency as the first electrical signal.
 16. The system ofclaim 1, further including an amplifier, the amplifier electricallycoupled between the oscillator and the transmit transducer.
 17. A systemfor location outside of a biological body including a biologicalstructure containing an implant with a sampling area for the biologicalstructure, the biological structure having a fluid flow of an averagevelocity in the sampling area with surfaces traveling with substantiallythe average velocity in the fluid flow through the sampling area of thebiological structure, the implant configured to transmit a firstelectromagnetic signal and a second electromagnetic signal, the firstelectromagnetic signal based upon a first electrical signal with a firstfrequency, the second electromagnetic signal based upon a combinedelectrical signal of a related version of the first electrical signalhaving a selectively filtered frequency of the first frequency and arelated version of a second electrical signal having a selectivelyfiltered frequency of the second electrical signal, the secondelectrical signal having substantially the frequency of the firstelectrical signal substantially Doppler shifted according to the averagevelocity in the fluid flow when the implant is implanted in thebiological structure, the system comprising: a receiver assemblyconfigured to receive the first electromagnetic signal to outputsubstantially the first electrical signal and to receive the secondelectromagnetic signal to output substantially the combined electricalsignal; and a frequency transformation module electrically coupled tothe receiver assembly, the frequency transformation module configured tooutput an output related to the average velocity of the fluid flowthrough the biological structure in the sampling area of the implant.18. The system of claim 17 wherein the frequency transformation moduleis configured to output a frequency domain Doppler spectrum based uponthe first electrical signal and the combined electrical signal, theDoppler spectrum related to the average velocity of the fluid flowthrough the biological structure in the sampling area of the implant.19. The system of claim 17, further including a VCO electrically coupledto the receiver assembly to receive the first electrical signal, the VCOconfigured to generate a substantially in-phase signal associated withthe first electrical signal and the related version of the firstelectrical signal and to generate a substantially quadrature signalassociated with the first signal and the related version of the firstelectrical signal.
 20. The system of claim 19, further including a firstmixer coupled to the receiver assembly and the VCO to combine thecombined electrical signal and the in-phase difference signal andincluding a second mixer coupled to the receiver assembly and the VCO tocombine the combined electrical signal and the quadrature differencesignal, the frequency transformation module being electrically coupledto the first mixer and the second mixer.
 21. The system of claim 17wherein the receiver assembly includes a first receiver configured toreceive the first electromagnetic signal and includes a second receiverconfigured to receive the second electromagnetic signal.
 22. The systemof claim 17 wherein the receiver assembly includes a receiver, and afirst bandpass filter electrically coupled between the receiver and thefrequency transformation module and a second bandpass filterelectrically coupled between the receiver and the frequencytransformation module, the first bandpass filter configured to outputsubstantially the first electrical signal and the second bandpass filterconfigured to output substantially the combined electrical signal.
 23. Asystem comprising: an implant for implantation into a biologicalstructure of a biological body having a fluid flow of an averagevelocity with surfaces traveling with substantially the average velocityin the fluid flow through the biological structure, the implantcomprising: an oscillator configured to generate a first electricalsignal having a first frequency; a transmitter assembly configured totransmit a first electromagnetic signal out of the biological structurewhen the implant is implanted into the biological structure, the firstelectromagnetic signal having a frequency that is based upon selectivefiltering of the first frequency of the first electrical signal; a mixerconfigured to output a combined electrical signal of a related versionof the first electrical signal having a selectively filtered frequencyof the first frequency and a related version of a second electricalsignal having a selectively filtered frequency of the second electricalsignal, the second electrical signal having substantially the frequencyof the first electrical signal substantially Doppler shifted accordingto the average velocity in the fluid flow when the implant is implantedin the biological structure; and the transmitter assembly furtherelectrically coupled to the mixer to output a second electromagneticsignal based on the combined electrical signal; and an extraluminalcomponent for location outside of the biological body, the extraluminalcomponent including: a receiver assembly configured to receive the firstelectromagnetic signal to output substantially the first electricalsignal and to receive the second electromagnetic signal to outputsubstantially the combined electrical signal; and a frequencytransformation module electrically configured to output an outputrelated to the average velocity of the fluid flow through the biologicalstructure in the sampling area of the implant.
 24. The system of claim23 wherein the frequency transformation module is configured to output afrequency domain Doppler spectrum based upon the first electrical signaland the combined electrical signal, the Doppler spectrum related to theaverage velocity of the fluid flow through the biological structure inthe sampling area of the implant.
 25. The system of claim 23 wherein theselectively filtered frequency of the first frequency is substantiallythe first frequency.
 26. The system of claim 23 wherein the selectivelyfiltered frequency of the first frequency is substantially a multiple ofthe first frequency.
 27. The system of claim 23 wherein the selectivelyfiltered frequency of the first frequency is substantially a fraction ofthe first frequency.
 28. The system of claim 23 wherein the extraluminalcomponent further includes a VCO electrically coupled to the receiverassembly to receive the first electrical signal, the VCO configured togenerate a substantially in-phase signal relating to the firstelectrical signal and the related version of the first electrical signaland to generate a substantially quadrature signal relating to the firstsignal and the related version of the first electrical signal.
 29. Thesystem of claim 28 wherein the extraluminal component further includinga first mixer coupled to the receiver assembly and the VCO to combinethe combined electrical signal and the in-phase difference signal andincluding a second mixer coupled to the receiver assembly and the VCO tocombine the combined electrical signal and the quadrature differencesignal, the frequency transformation module being electrically coupledto the first mixer and the second mixer.
 30. The system of claim 23wherein the receiver assembly includes a first receiver configured toreceive the first electromagnetic signal and includes a second receiverconfigured to receive the second electromagnetic signal.
 31. The systemof claim 23 wherein the transmitter assembly has a first transmitterunit and a second transmitter unit, the first transmitter unitconfigured to transmit the first electromagnetic signal and the secondtransmitter unit configured to transmit the second electromagneticsignal.